1. Field of the Invention
This invention relates to a high power gas discharge tube of novel characteristics, and to applications of the tube in the control of electric motors and other alternating current devices.
2. Review of the Art
As the current passed through a gas discharge tube is increased beyond the levels at which normal glow discharge takes place, such normal gas discharge being characterized by a negative resistance characteristic leading to decreasing potential between the cathode and anode electrodes of the tube, a region of abnormal glow discharge is entered in which the negative resistance characteristic changes to a positive resistance characteristic leading to increasing potential between the electrodes. Typically this increased potential rapidly leads to breakdown into vacuum arc discharge between the electrodes, again characterized by a negative resistance characteristic. Accordingly, gas discharge tubes have been operated in the normal glow discharge or vacuum arc regimes in which stable operation can be achieved by suitable ballasting of the tube, the former regime being suitable for low current applications and the latter for high current. It is possible to utilize a normal glow discharge tube in a low frequency oscillator circuit by placing capacitance in parallel with the tube and in series with the ballast because such a tube is characterized by a comparatively high striking potential at which discharge is initiated, and a lower but still high extinction potential at which discharge ceases. Operation in such a mode with vacuum arc devices is difficult because, in order to turn off the device effectively, the arc must be extinguished or otherwise interrupted or divested for long enough to disperse the intense ionization formed in its path.
Devices operating in the vacuum arc regime have other problems, particularly in terms of ensuring adequate electrode life, which have led to gas diodes and triodes (thyratrons) being superseded by semiconductor devices in most applications. A further limitation of such devices is that the great difficulty in turning them off, except by terminating current flow through the device for a finite period, limits their usefulness as control devices to rectification, current turn-on and low frequency alternating current applications.
The only prior art of which we are aware which successfully exploits the abnormal glow discharge regime is the process described in U.S. Pat. No. 3,471,316 (Manuel) issued Oct. 7, 1969, which we understand is commercially utilized in forming organic coatings on metal cans. It relies on the application of externally generated current pulses to force a discharge tube temporarily into the abnormal glow discharge region, the pulses being sufficiently short that no vacuum arc is established. There is no disclosure of any endogenous pulsed abnormal glow discharge, the apparatus is dependent upon an external pulse generator to operate, and its utility is completely different from the present invention because it uses externally generated pulses rather than generating such pulses.
We are also aware that the use of vacuum arc discharge tubes has been proposed for the control of inverters, as exemplified by U.S. Pat. No. 4,194,239 (Jayaram et al), which discloses the use of vacuum arc discharge tubes in which the discharge is steered magnetically between multiple electrodes to provide a commutating effect. Such an arrangement acknowledges the difficulty of extinguishing a vacuum arc, and seeks to overcome the difficulty by instead switching the discharge between electrodes by the use of externally applied magnetic fields.